Over the last few decades, biodegradable polymers have been applied to a number of applications in drug delivery and regenerative medicine. While naturally derived biodegradable polymers have distinct bioactivity and cell binding properties, they are difficult to isolate, derivatize and purify. Synthetic polymers also have the potential for immunogenic responses. Synthetic biodegradable polymers have a number of advantages over natural materials, especially the chemical diversity of monomers that can be utilized to tailor the chemical, mechanical and degradation properties of the polymer. There are a number of biodegradable polymers including poly(ε-caprolactone) (PCL), poly(lactic acid) (PLA), poly(glycolide) (PGA), and copolymers thereof that are used clinically and while their properties in vitro and in vivo are largely understood, their range of physical and chemical properties is somewhat limited. Efforts have been made to diversify the pool of synthetic polymers to meet design criteria for more advanced applications. Currently, a wide range of polymers including polyurethanes, polycarbonates and poly(α-amino acids) have been utilized in biomedical and regenerative applications.
More recently, amino acid-based poly(ester urea)s (PEUs) have been studied for use in various biomedical and regenerative applications. The use of specific amino acids influences the physical and chemical properties of the resulting polymers and also provides significant variability in the chemical structures. Amino acid-based PEUs are semi-crystalline, and thermal or solution-based processing methods offer non-chemical routes to tune their mechanical properties, chemical stability and degradation rates. To further enhance the biological interactions, there has been a focus on attaching bioactive molecules to these polymers. Conventionally, the bioactive molecules are attached to the PEU polymer prior to processing the PEU into a desired form or configuration for use. However, attaching bioactive molecules like peptides or proteins prior to processing is generally difficult and the biological activity is often lost due to denaturation or degradation.
The incorporation of reactive sites into biodegradable polymers provides a platform for the conjugation of biological cues. To meet the challenge of regiospecific biomolecular derivation, it is particularly attractive to employ orthogonal “click” chemistry methods. The “click” concept currently represents a number of reactions, which are robust, selective, efficient, and high yielding. The catalog of “click” reactions includes copper (I) catalyzed azide-alkyne cycloaddition (CuAAC), thiol-ene radical addition, oxime ligation, Michael-addition, among others. They are widely utilized for protein and DNA conjugation, cell modification, surface functionalization, and in vivo signaling. Other “click” reactions, such as thiol-maleimide and NHS-ester coupling, are also widely used in the fields of material and life science.
Recently, “click” chemistry has been used for protein and peptide conjugation to tyrosine-based phenol residues using both Mannich-type addition and “ene-type” addition reactions. Compared to the large abundance of lysine residues typically found in proteins, the tyrosine content is much lower. In addition, unlike the disulfide linkages and bridges enabled by cysteine residues in close proximity, tyrosine is available for chemical modification without additional protection/deprotection steps.
What is needed in the art is a novel tyrosine-based PEUs functionalized specifically to bond with bioactive molecules via a click reaction after the PEU has been processed into its intended final form and/or composition and related methods of attaching bioactive molecules to the tyrosine-based PEUs using “click” chemistry reactions.